Antenna diversity is a wireless diversity scheme that uses two or more antennas to improve the quality and reliability of a wireless link. Currently 3GPP (3rd Generation Partnership Project) is evaluating uplink (UL) transmission diversity techniques to enhance the uplink capacity and UE (user equipment) power consumption. One of the techniques is referred to as Switched Antenna TX Diversity (SATD), where the UE transmits from one of two different antennas at each time instance. The gain from SATD stems from the diversity gain that is experienced due to the different transmit antennas, and by using the antenna with higher gain, lower transmission power is needed to reach the desired SIR (signal-to-interference ratio) target. For an interference-limited system, lower transmission power results in lower interference between cells (i.e. inter-cell interference), which may lead to higher cell throughput. For a power limited UE, the diversity gain corresponds to coverage gain.
An open loop SATD algorithm typically selects transmitting antennas based on TPC (transmit power control) commands e.g. by switching antennas when a cumulative TPC sum is greater than a target sum (STPC) and/or a antenna switch interval based switch e.g. by always switching after Tf-s consecutive transmissions on the same antenna. TPC commands can be used to determine which UE antenna is preferable, e.g. TPC down commands after switching from one antenna to another imply that a better antenna has been chosen while TPC up commands imply that an inferior antenna has been selected. Several TPC up commands while on the same antenna also implies that the antenna gain is deteriorating and that a switch may be beneficial.
The use of a timer-based switch ensures the UE is not relegated to using an inferior antenna. This can happen when the transmission conditions associated with the selected antenna are fairly constant while the transmission conditions for the other antenna are improving. Without a timer-based switch it is difficult to discover that the other antenna is better than the selected antenna.
Optimal values of the parameters for both the TPC-based switch algorithm and the timer-based switch algorithm depend on channel conditions, user speed and antenna implementation. Often the channel conditions and/or gain can differ significantly between antennas e.g. by up to 10 dB. For example, a fixed imbalance between UL transmit antennas of 0 dB or 4 dB has been used in 3GPP evaluations. Added to the fixed imbalance is a random (per UE) imbalance with standard deviation of 2.25 dB in 3GPP evaluations.
As such, it is not uncommon or unrealistic that there is a large difference between the gains on the different antennas. The conventional UL SATD algorithms described above can perform well under certain conditions. Accordingly, transmission power can be decreased efficiently. However, these conventional UL SATD algorithms are not designed for optimizing cell throughput. As such, the data rate never reaches its possible maximum level.